Retinoblastoma is the most common intraocular tumor in childhood and its origin is in the embryonal layer, the neuroectoderm. It shares some histopathological and biological features with other neoplasms of the central nervous system (neuroblastoma). Flexner-Wintersteiner rosettes and fleurettes in differentiated retinoblastoma was interpreted as photoreceptor cell differentiation, however, no molecular and biochemical markers have yet been described supporting this hypothesis. Our in vitro culture system allows for the routine growth and amplification of primary retinoblastoma cells. We documented by light and electron microscopy the differentiation of retinoblastoma into Flexner-Wintersteinder rosettes in vitro. It is the goal of this proposal to characterize retinoblastoma with cell and tissue specific markers (intermediate filaments), retina specific markers (rhodopsin and transducin) and embryonic regulatory markers (homeotic genes) using our established culture system. Expression of GFAP will demonstrate glial cell and astrocyte differentiation in retinoblastoma previously suggested. Analysis of expression of rhodopsin and/or transducin using cDNA probes will for the first time demonstrate photoreceptor cell specific markers in this tumor and therefore support the original hypothesis. Homeotic genes are regulatory genes whose activities are vital for normal pattern formation, embryonic development and cell differentiation. We hypothesize that these regulatory genes like the N-myc oncogene are involved in the penetration of the cancer phenotype in this embryonal tumor. We will compare the expression or homeotic genes and N-myc oncogene in differentiating retinoblastoma and the developing retina in order to establish correlations between oncogene and homeotic gene expression and their roles in the development of neural crest tumors.